gdb.base/examine-backward.exp C++ify and Clang

gdb.base/examine-backward.exp C++ify and Clang

Adjust gdb.base/examine-backward.exp to let the testcase build and run
as a C++ program, built with either G++ or Clang++.

The change to use unsigned char instead of plain char is to avoid
narrowing warnings:

  gdb compile failed, src/gdb/testsuite/gdb.base/examine-backward.c:55:1: error: narrowing conversion of '227' from 'int' to 'char' [-Wnarrowing]
     55 | };
	| ^

gdb/testsuite/ChangeLog:

	* gdb.base/examine-backward.c (Barrier, TestStrings): Now unsigned
	char array.
	(main): Add references to Barrier, TestStrings, TestStringsH and
	TestStringsW.
	* gdb.base/examine-backward.exp: Issue "set print asm-demangle on"
	and expect a full prototype in C++.